1. Field of the Invention
The invention relates to processes for fabrication of semiconductor devices, and in particular to the fabrication of metal-nitride-silicon (MNS) devices.
2. Description of the Prior Art
Metal-nitride-oxide-semiconductor (MNOS) structures are well known in the prior art, such as in U.S. Pat. No. 3,419,761, particularly for the fabrication of insulated gate field effect transistors. Silicon nitride (Si.sub.3 N.sub.4, hereafter referred to simply as "nitride") as a gate dielectric for an MIS device has a number of desirable properties, including a high dielectric constant (6.2.epsilon..sub.o) and a high dielectric breakdown strength as compared to silicon dioxide (SiO.sub.2, hereafter referred to simply as "oxide"). During early days of MIS device processing, it was recognized that the presence of nitride over oxide as a dual dielectric structure minimized ionic contamination of the oxide layer. The dual dielectric structure, however, did exhibit bias-temperature (B-T) instability.
Other techniques in which the nitride layer is deposited directly over silicon by a chemical vapor deposition (CVD) or reactive silane-ammonia plasma technique results in a porous structure with a large hysteresis in the capacitance-voltage characteristics of the resultant MIS structure, not suitable for the gate electrode in high performance FET devices. It has been reported recently (reference 1) that one can convert the surface of silicon into a nitride if the silicon wafer is heated in nitrogen or NH.sub.3 to 1200.degree.-1300.degree. C. The growth rate of the nitride has been reported (reference 2) to vary as a function of time and a maximum growth rate of 2.8 A/sec has been achieved in N.sub.2 at 1250.degree. C. after a 120 seconds exposure. The nitride film obtained by this technique exhibits an index of refraction of .perspectiveto.2-2.3. The direct thermally grown nitride films of .perspectiveto.75-100 A film thickness have also been shown to act as very efficient oxidation and diffusion masks. In addition the nitride film can be used as a gate dielectric layer where the density of surface state at the nitride-silicon surface is low (.perspectiveto.3.times.10.sup.10 cm.sup.-2 ev.sup.-1) and the channel electron mobility is high (.perspectiveto.800 cm.sup.2 /v-sec).
Direct thermally grown nitride has been used in the prior art to fabricate IGFET type devices such as described in reference 1. A 95 A nitride layer, grown in NH.sub.3 at 1000.degree. C., was used as a gate dielectric. The electrical properties of the nitride layer, however, have been shown to be sensitive to the level of O.sub.2 contamination in the p.p.m. range. The purity of NH.sub.3, in particular the H.sub.2 O contamination, is believed to exhibit considerable variation depending upon the source and the ambient conditions which makes such processes disadvantageous.
Attempts have also been made in the prior art to nitridate silicon wafers in N.sub.2, such as described in reference 2. This, however, requires one to raise the silicone substrate temperature to greater than 1300.degree. C. In many instances such a high temperature process step is not compatible with the substrate, such as silicon on sapphire (SOS), due to the introduction of impurities into the active silicon layer from the underlying substrate. In some cases this high temperature step might cause formation of additional lattice defects in the silicon.
The high nitridation temperature limits the use of the above process in VLSI and VHSI circuit processing due to the following effects:
(a) thermal distortion of the silicon substrate which would require large alignment tolerance and hence less circuit packing density. PA0 (b) excessive thermal outdiffusion of elements from the sapphire substrate into the silicon in case a silicon-on-sapphire substrate is used. PA0 (c) increased redistribution of dopants in the silicon substrate. PA0 1. T. Ito, T. Nozski, H. Arakawa and M. Shinoda, "Thermally Grown Silicon Nitride Films for High-Performance MNS Devices," Appl. Phys. Lett., 32, 330, 1978 PA0 2. T. Ito, S. Hijiya, T. Nozaki, H. Arakawa, M. Shinoda and Y. Fukukawa, "Very Thin Silicon Nitride Films Grown by Direct Thermal Reaction with Nitrogen", Jour. of Electrochem. Soc., 125, No. 3, 448 (1978) PA0 3. T. Ito, S. Hijiya, H. Ishikawa and M. Shinoda, "10V Write/Erase, EAROM Cells with Directly Nitrided Silicon Nitride Films As First Insulating Layers", IEEE IEDM Technical Digest, p. 284, Washington, D.C., December 1977 PA0 4. M. J. Rank, Ext. Abst #193 ECS Fall Mtg., Atlanta 1977
Prior to the present invention there has not been a process of direct thermal nitridation of silicon suitable for fabrication of semiconductor devices.